Polyethylene to see more use during switch to 1-piece caps

In the U.S., processors use injected molded, high density polyethylene closures to cap water bottles. However, carbonated soft drinks and hot-fill applications are usually topped with compression molded, two-piece closures — an outer polypropylene shell and a liner, usually made from ethylene vinyl acetate.

The U.S. preference for two-piece caps is unique — one-piece closures are common on soft drinks in other locations, primarily Europe and Asia — and the market’s tastes are starting to shift, according to experts who spoke at Plastics News’ Caps & Closures 2012, held Sept. 18-19 in Lincolnshire.

One-piece closures “will be the next big thing to hit the U.S.,” said Mike Sansoucy, president of Netstal Machinery Inc. in Devens, Mass.

Improved materials and closures design has led to the creation of one-piece caps that offer the same sealing surface as two-piece closures, allowing the caps to be used in carbonated soft drink and hot-fill applications, Sansoucy said.

Switching from a two-piece to a one-piece closure requires changing both material and manufacturing process, and, according to the experts, these changes offer several economical and environmental advantages.

PE, made of natural gas feedstock, is now a more cost-effective material than PP. The material also has a balance of toughness and stiffness that can make it a prime replacement for PP in most applications, said Eric Vignola, research and development manager for PE products at Nova Chemicals Corp. of Calgary, Alberta.

And processors are taking advantage of those opportunities — HDPE is the fastest-growing material in the caps and closures arena and it’s taking some of that market share from PP, partly thanks to the growing one-piece market, said Ashish Chitalia, project manager at Houston-based Chemical Market Resources Inc.

Since making the switch eliminates the need for an EVA liner, the caps are easier to recycle and the one-step process makes manufacturing more sustainable, Sansoucy said.

“If you can injection mold the cap finished, that’s that. You don’t have to add an extra step, so just from a manufacturing standpoint, it’s a greener solution,” he said.

Sustainability has been an ongoing trend in the caps and closures market, and has driven machine manufacturers to create smaller systems for smaller, strategically located plants, Sansoucy said.

It has also pushed the creation of increasingly lighter caps and closures. In five years, single-piece water-bottle caps have shrunk from an average of 1.45 grams to an average of 0.85 gram, while carbonated soft drink caps have gone from 4.5 grams on average to 2.65 grams, he said.

“I don’t know how much lighter you can go on a still-water cap … before you’ve just got a bag with a flap on top of it,” Sansoucy joked.

Switching to one-piece caps can eliminate material in both the cap and the bottle, said Lothar Brauer, marketing and business development director at Bericap GmbH & Co. KG.

To support both the weight of a two-piece cap and the torque needed to seal it, bottles used in hot-fill applications need to have heavier and more robust neck finishes — typically about 8.2 grams, Brauer said.

One-piece closures, like the ones designed by Bericap, are lighter weight, feature a large seal angle and are designed to require less torque, so neck finishes can be lighter, he said. He added that replacing two-piece caps with one-piece caps designed by Bericap could reduce resin by 6 tons and carbon output by 17 tons per 500 million bottles.

But the switch to one-piece closures isn’t necessarily an easy one.

The move from PP to PE can be challenging, Nova’s Vignola said. While there’s a wealth of knowledge about using PP in caps and closures applications, there isn’t the same awareness about PE, he added.

Comparing the properties of a PP resin and PE resin can be tricky because the two materials “don’t speak the same language,” he said.

For example, flow in PP resins is measured as melt-flow rate while in PE resins it’s measured as melt index, he said. “The dangerous thing here is that they’re measured at different temperatures, so if you’re comparing two data sheets, you’re not comparing apples to apples.”

Going to injection molded one-piece caps also means an entirely new production system, Sansoucy said.

The transition will require the entire supply chain to get involved. The liner in two-piece caps covers “a multitude of sins on the bottle side,” so processors will need to ensure they’re making bottles correctly, he said.

Even with the hurdles, brands will probably start switching to one-piece caps in the next year or so, Sansoucy said. “It’s going to take brand owners, and maybe it’s not the big players but the smaller players who say we’re willing to risk, or we think it’s time to make that switch away from a lined cap,” he added.